As dimensions of fabricated polygons of semiconductors continue to shrink, it is becoming very difficult to produce regular, rectangular polygons because refraction and other optical effects may alter the expected shape of a polygon. In the past, with an in-house fab, a semiconductor vendor was able to obtain Scanning Electron Microscope (SEM) pictures of fabricated polygons for the purpose of calibrating an aerial image simulator so that the fabrication process may be characterized and the contours of fabricated polygons may be predicted by simulation. However, as the semiconductor vendors are becoming fabless, such SEM calibration data is extremely expensive, if obtainable at all. Without such calibration data, aggressive scaling in a particular process technology often results in low yields, costing the vendors lots of money. On the other hand, if the process technology is not used to its full potential (non-aggressive scaling), then the resulting fabricated chips may be large and have poor performance.
There are many known problems that make it difficult to reliably fabricate the polygons of a semiconductor to create a functioning chip. It has become common practice to perform Optical Proximity Correction (OPC) on the polygons described in a Geographic Data System (GDS) file. Essentially, OPC alters the GDS polygons based on a function ƒ−1(x) that is the inverse of the function ƒ(x) which describes the fabrication process, such that when the a particular OPC'ed polygon is fabricated, the resulting contour closely resembles that described in the GDS file. A common example is adding “hammerheads” to a polygon to prevent “line-end shortening,” a phenomenon wherein the length of a polygon is severely truncated due to refractive and other optical effects during fabrication.
However, OPC does not result in fabricated polygons that always closely resemble their respective GDS descriptions. Thus, margins are placed in the design rules to account for these discrepancies, resulting in a set of rules that do not permit the capabilities of a particular process technology to be fully utilized.
To be competitive in the marketplace, it is very important to be able to fine tune the design rules to reduce as much of the margins as possible. For a fabless company, however, this is almost impossible due to the lack of fabrication data.
Thus, it is desirable to be able to characterize the fabrication process of a particular process technology without relying on the very expensive or even impossible-to-acquire SEM data, and to provide a solution wherein a fabless company may optimize design layouts based on more finely tuned design rules by characterizing the fabrication process without requiring SEM or other analog data measurements.